As a blue-violet semiconductor laser has been put into practical use, a Blu-ray Disc (hereinafter, called as BD), as a high-density and large-capacity optical information recording medium (hereinafter, also called as an optical disc) and having substantially the same size as a CD (Compact Disc) and a DVD (Digital Versatile Disc), has been put into practical use. The BD is an optical disc having a light transmissive layer of a thickness of about 0.1 mm, and is adapted to record or reproduce information, using a blue-violet laser light source for emitting blue-violet light of about 400 nm wavelength, and an objective lens whose numerical aperture (NA) is about 0.85.
In recent years, forming a multilayer structure of information recording surfaces in a high-density optical disc such as a BD to be used with a blue-violet semiconductor laser has been conceived to further increase the capacity of the disc. In the case where plural layers of information recording surfaces are formed, information is recorded or reproduced with respect to each of the information recording surfaces. In this arrangement, since the thicknesses of the light transmissive layers are different from each other among the information recording surfaces, a third-order spherical aberration is generated on an information recording surface having a layer thickness different from an optimum light transmissive layer thickness (the thickness of a light transmissive layer which minimizes a third-order spherical aberration) with respect to the objective lens, depending on the distance from the light transmissive layer having the optimum layer thickness to the information recording surface.
For instance, in the case where the wavelength of light to be emitted from a semiconductor laser is 400 nm, the NA of the objective lens is 0.85, and the thickness of a light transmissive layer is displaced from the optimum light transmissive layer thickness by 10 μm, a third-order spherical aberration of about 100 mλ is generated. In view of the above, generally, an optical head for the optical disc having the above structure is provided with means for correcting a third-order spherical aberration.
For instance, patent literature 1 discloses an optical disc device, wherein a collimator lens is loaded on a collimator lens actuator, and the collimator lens disposed between a light source and an objective lens is moved in such a direction as to cancel a third-order spherical aberration resulting from a thickness difference in the light transmissive layer, whereby the divergent angle or the convergent angle of laser light to be incident into the objective lens is changed.
Further, many of the optical heads for a high-density optical disc and using short-wavelength laser light and an objective lens having a large numerical aperture are provided with means for correcting a third-order coma aberration resulting from a tilt (hereinafter, also called as a disc tilt) of the optical disc. In the optical heads, for instance, a method of tilting an objective lens loaded on an objective lens actuator, or a method using a liquid crystal element has been put into practical use.
In the case where a multilayer structure of information recording surfaces is formed, and a spherical aberration is corrected depending on the thickness of the respective light transmissive layers of an optical disc, it is well known that the aberration amount of third-order coma aberration resulting from a disc tilt, and the aberration amount of third-order coma aberration resulting from a tilt (hereinafter, also called as a lens tilt) of an objective lens are respectively changed depending on the thickness of the respective light transmissive layers of the optical disc.
For instance, patent literature 2 discloses an optical disc device, wherein the magnitude of a drive signal of coma aberration correcting means is changed depending on a ratio between the aberration amount of third-order coma aberration resulting from a disc tilt, and the aberration amount of third-order coma aberration resulting from a lens tilt. The conventional optical disc device disclosed in patent literature is described referring to FIG. 17.
FIG. 17 is a diagram showing a schematic arrangement of a conventional optical head. Referring to FIG. 17, an optical head 150 includes a light source 101, a diffraction grating 102, a polarized beam splitter 103, a front monitor sensor 104, a collimator lens 105, a beam expander 106, a quarter wavelength plate 107, a mirror 108, an objective lens actuator 109, an objective lens 110, a detection lens 112, and a light receiving element 113. An optical disc 111 has a first layer 111a formed on a side opposite to a light incident side of the optical disc 111, and a second layer 111b formed on the light incident side thereof.
Laser light emitted from the light source 101 is separated into three beams by the diffraction grating 102, transmitted through the polarized beam splitter 103, and incident into the collimator lens 105. A part of the laser light incident into the polarized beam splitter 103 is reflected on the polarized beam splitter 103, and incident into the front monitor sensor 104. The output of the light source 101 is controlled based on the output from the front monitor sensor 104.
The laser light incident into the collimator lens 105 is converted into substantially parallel light, and incident into the beam expander 106. A convex lens constituting the beam expander 106 is loaded on an actuator (not shown), and is movable in the optical axis direction to change the divergent angle or the convergent angle of laser light to be incident into the objective lens 110.
The laser light transmitted through the beam expander 106 is converted into circular polarized light while being transmitted through the quarter wavelength plate 107, reflected on the mirror 108, and incident into the objective lens 110. The laser light which is converged by the objective lens 110, and incident into an information recording surface of the optical disc 111 is reflected on the information recording surface of the optical disc 111, transmitted through the objective lens 110, and then reflected on the mirror 108. The laser light reflected on the mirror 108 is incident into the quarter wavelength plate 107, converted into linear polarized light whose polarization direction is rotated by 90 degrees with respect to the polarization direction on the outward path, and transmitted through the beam expander 106. The laser light transmitted through the beam expander 106 is converged by the collimator lens 105, and reflected on the polarized beam splitter 103. The laser light reflected on the polarized beam splitter 103 is incident into the light receiving element 113 through the detection lens 112, whereby an RF signal and a servo signal (a focus error signal and a tracking error signal) are detected.
A spherical aberration resulting from a thickness difference between the light transmissive layers of the first layer 111a and the second layer 111b of the optical disc 111 can be corrected by generating a spherical aberration having a polarity opposite to the polarity of the spherical aberration by converting the laser light to be incident into the objective lens 110 into divergent light or convergent light by the beam expander 106.
The objective lens actuator 109 drives the objective lens 110 in such a manner that a light spot follows an information track on the rotating optical disc 111, using a focus error signal and a tracking error signal. The objective lens actuator 109 is operable to tilt the objective lens 110 in the radial direction of the optical disc 111.
In the above arrangement, if a third-order spherical aberration is corrected with respect to the first layer 111a and the second layer 111b of the optical disc 111 depending on the thickness of the respective light transmissive layers, the aberration amount of third-order coma aberration resulting from a disc tilt, and the aberration amount of third-order coma aberration resulting from a lens tilt are different from each other. In view of the above, in the conventional optical head 150 disclosed in patent literature 2, a lens tilt amount is optimized by setting the lens tilt amount with respect to the second layer 111b to a predetermined amount smaller than the lens tilt amount with respect to the first layer 111a to thereby allow stable recording and reproducing operations at the time of correcting a third-order spherical aberration.
In the case where plural layers of information recording surfaces are formed, since the thicknesses of the light transmissive layers are different from each other among the information recording surfaces, a third-order spherical aberration is corrected by converting laser light to be incident into an objective lens into divergent light or convergent light.
In the above arrangement, the aberration amount of third-order coma aberration resulting from tilting an objective lens is different among the information recording surfaces. As the thickness of a light transmissive layer is decreased, the aberration amount of coma aberration resulting from tilting an objective lens by a predetermined angle is increased, and as the thickness of the light transmissive layer is increased, the aberration amount of coma aberration resulting from tilting the objective lens by a predetermined angle is decreased.
On the other hand, the aberration amount of coma aberration resulting from tilting an optical disc is increased in proportion to the thickness of a light transmissive layer. As the thickness of a light transmissive layer is decreased, the aberration amount of coma aberration resulting from tilting an optical disc by a predetermined angle is decreased, and as the thickness of a light transmissive layer is increased, the aberration amount of coma aberration resulting from tilting the optical disc by a predetermined angle is increased.
Accordingly, in the case where the thickness of a light transmissive layer is small, the tilt angle (the lens tilt angle) of an objective lens for correcting a coma aberration resulting from a disc tilt is small. However, as the thickness of a light transmissive layer is increased, the tilt angle (the lens tilt angle) of an objective lens for correcting a coma aberration is sharply increased. Further, astigmatism is generated in addition to a coma aberration, if the objective lens is tilted. A third-order astigmatism is sharply increased, as the lens tilt angle is increased, and an influence of the astigmatism becomes non-negligible.
On the other hand, as the thickness of a light transmissive layer is decreased, a third-order coma aberration resulting from a lens tilt is sharply increased. As a result, in the case where an objective lens is tilted over an allowable range due to e.g. a lens tilt control error at the time of correcting a third-order coma aberration, resonance of an objective lens actuator, or a like occasion, a residual third-order coma aberration becomes non-negligible.
As BDs, a single-layer disc having a single information recording surface with a light transmissive layer of 100 μM in thickness, and a dual layer disc having two information recording surfaces with light transmissive layers of 100 μm and 75 μm in thickness are available. It is necessary to secure a certain distance between information recording surfaces of an optical disc having the information recording surfaces in order to suppress an influence by reflection light reflected on an adjacent information recording surface e.g. crosstalk between information signals, or an offset of a servo signal resulting from stray light reflected on an adjacent information recording surface. In view of the above, it is required to increase the distance between an information recording surface having a largest light transmissive layer thickness, and an information recording surface having a smallest light transmissive layer thickness in a multilayer optical disc having three or more layers, as compared with a conventional dual layer BD.
The aforementioned problem becomes more serious in an optical head for recording or reproducing information with respect to a multilayer optical disc having three or more layers, as compared with an optical head for recording or reproducing information with respect to a conventional optical disc. However, the conventional optical head and the conventional optical disc device do not mention the above problem.